High density lipoproteins (HDL) are lipid-protein complexes found in the circulation, that play vital roles in lipid transport. They have been the focus of much interest because of the finding that their concentration in the blood correlates inversely with the risk of atherosclerotic vascular disease. Hence, there is an urgent need to understand how HDL enters and leaves the circulation. This application is aimed at identifying the nature of HDL released by the liver, which together with the intestine is a major site of HDL production. Current evidence suggests that nascent HDL is secreted by the liver in an immature form and is then modified in the circulation to produce a "mature" HDL particle. By understanding this process one can begin to ask meaningful questions about how it is regulated, how it may be disturbed and what the consequences would be for HDL metabolism and lipid transport. Since nascent HDL precursors are not identifiable in normal serum or plasma the research plan calls for the collection of this material in a liver perfusion system. The baboon liver has been chosen for this purpose because of the similarity between the HDL of this animal and that of man. Perfusion media and liver homogenates will be analyzed by gel-filtration and density gradient ultracentrifugation and the distribution of nascent HDL determined using sensitive, specific radioimmunoassays for the two major HDL peptides or apoproteins, apo A-I and apo A-II. Newly-synthesized apoprotein will be recognized by isoelectric focusing and by the invorporation of radioactive tracers. Characterization and isolation of nascent hepatic HDL will be followed by in vitro studies of its maturation. A systematic approach in which nascent particles are incubated with various plasma factors considered to be potentially important in HDL assembly will identify the critical components of this process.